Currently, there are introduced several methods for measuring a distance to an object by using an ultrasonic wave in an air. Representative examples of the distance measuring method employing the ultrasonic wave are, for example, a pulse echo method, a phase angle method, a frequency modulation method and a correlation method. Among those methods, the pulse echo method is the most simple and has been adopted for long period of time in many areas such as a ranging sensor in a mobile robot and an intelligent traffic system.
Herein, therefore, there will be described a method which is substantially related to the pulse echo method. In the pulse echo method, an ultrasonic pulse having a frequency ranging from about 20 kHz to about 100 kHz is generated and transmitted to an object at time T0. Then, the ultrasonic pulse is reflected from the object, thereby an echo pulse of the ultrasonic pulse being detected by a sensor at time T1. In this regard, a propagation time of the pulse can be defined to be (T1−T0) and, accordingly a distance to the object can be given by a half of a multiplication of the propagation time and a velocity of an ultrasonic wave c, i.e., (0.5×(T1−T0)×c), wherein a velocity of an ultrasonic wave c is a known value.
One of good reasons to adopt an ultrasonic wave having a frequency ranging from 20 kHz to 100 kHz is to implement a high directivity of a ultrasonic pulse in the air. Generally, when a piston-shaped ultrasonic wave generator having a radius a harmonically oscillates with a frequency set to f, an ultrasonic wave beam propagates through the air with a form of a nearly planar wave in a near field. However, the beam becomes spread wide, thereby having a form of a circular cone in a far field, by a diffraction thereof in proportional to a propagating distance. Accordingly, a beam width becomes larger as the wave propagates farther from the wave generator and, consequently, an angle is formed between an outermost sideline of the propagating beam and a central direction line of the propagation. Such formed angle is defined as an angle of divergence θ can be defined as:θ=sin−1[0.61×c/(f×a)]  Eq. 1,as disclosed in “Fundamentals of Acoustics”, (4th edition, Wiley, New York, 1982 authored by L. Kinsler, A. Frey, Coppens and J. Sanders), wherein c is the velocity of an ultrasonic wave, f is the frequency of the ultrasonic wave and a is the radius of the ultrasonic wave generator.
In other words, the angle of convergence of the ultrasonic wave is inversely proportional to the frequency f and the radius α of the piston-shaped ultrasonic wave generator. As the angle of convergence becomes smaller, the beam width of the ultrasonic wave becomes narrower and, resultantly, a spatial resolution can be increased. Therefore, it is generally desirable to minimize the beam width to achieve a high resolution in a spatial domain.
Referring to Eq. 1 and the aforementioned relation between the angle of convergence and the beam width of the ultrasonic wave, the beam width is minimized by increasing the frequency f of the ultrasonic wave. However, the method of increasing the frequency of the ultrasonic wave has a drawback that a measurable range of a distance decreases, because the ultrasonic wave is attenuated in proportional to square of the frequency. Another method for minimizing the beam width is to increase the radius a of the piston-shaped ultrasonic wave generator. However, it is practically difficult to implement the larger radius of the piston-shaped ultrasonic wave generator mechanically. Furthermore, a size of a sensor therein becomes large in proportional to the diameter thereof. For the reasons stated above, the commonly used sensors has the radius a which is less than or equal to 15 mm, and measures the distance by using the ultrasonic wave at the frequency of 40 kHz.
Meanwhile, a directivity characteristic of the sensors can be represented with a half power beam width 2θHP (hereinafter, referred to as HPBW for simplicity). For example, for a commonly used sensor having the radius of 12 mm and using the frequency of 40 kHz, the HPBW is known to be about 20 degrees. In this case, the beam width of the wave becomes larger than 1 m at a 5 m distant place from the sensor. In this regard, although the beam width is also slightly dependent on other factors, e.g., duration of the pulse or a source type (piston source or Gaussian source), the sensor having the aforementioned directivity characteristic is generally called to have the spatial resolution of 1 m at a 5 m distant place from the sensor.
As described above, the conventional ultrasonic distance measuring method has a limitation in reducing the beam width of the ultrasonic wave pulse, in that a pulse having the high frequency is directly transmitted, reflected and detected for the ranging. Consequently, the conventional method has the drawback of poor resolution in the spatial domain.
Still another drawback of the conventional ultrasonic distance method is that a side lobe is formed in a direction other than the central direction of the propagation, while a main component of the ultrasonic pulse is propagating into the central direction thereof. The side lobe can be reflected to an unintended object such that an undesirable echo pulse, which is the reflected pulse of the side lobe, may be detected prior to detecting the reflected pulse of the main component thereof. Consequently, the side lobe causes a cross talk, thereby making it difficult to measure the distance to the target object precisely. Therefore, it is important to suppress the side lobe in a technology area where the ultrasonic sensors are applied.